The field device includes a control/evaluation unit having a main electronics, and a sensor having a sensor electronics. These are spatially separated from one another, and connected with one another via a connection means, usually a cable; alternatively, in the case of a compact device, the main electronics and the sensor electronics are arranged in one housing. The two electronics can be arranged on different circuit boards, or also on the same circuit board. Further provided are: at least one data line, via which the sensor and the control/evaluation unit communicate; and a power supply line, via which a power supply voltage sufficient for operation of the sensor is made available to the sensor.
The field device is preferably a fill level measuring device, a pressure measuring device, a flow measuring device or an analytical measuring device designed for purposes of analyzing a liquid or gaseous medium. This list is not, of course, intended to represent the limits of the definition; rather, with the term “field device” is meant a measuring device which makes available information concerning any physical or chemical, process variable.
The invention relates to cable probes, which are used when the measuring should be performed by means of a sensor—e.g. by means of a pressure sensor or a capacitive measuring probe—at a site which is not directly accessible externally. A typical example is the application of a probe/sensor at a certain height in a tank, or in some other container difficult to access from the outside. The connection means—that is the cable—serves for securement of the sensor in the container; via corresponding lines in the cable, the energy supply and the transmission of data between the evaluation unit and the sensor occur simultaneously.
From EP 1 228 494 B1, a corresponding apparatus for transmission of data between a sensor and an evaluation unit has been made known. Sensor and evaluation unit are separated from one another and have a certain spatial distance. Field devices embodied as cable variants are available from the firm, Endress+Hauser.
In order to eliminate disturbance/interference currents on the lines, the main electronics arranged in the evaluation unit and the sensor electronics located in the sensor are usually galvanically isolated from one another. For galvanic isolation of the lines, switching power supplies of different topology, such as push-pull converters, flyback converters or forward converters are used. In this way, the possibility exists to reduce possible interference currents in such a manner that the respective requirements of the EMC standards are fulfilled.
Problems become apparent in the case of galvanic isolation, when voltages of the same order of magnitude should be transmitted. A typical DC/DC transmission is, for example, the transforming of 3.3V to 3.3V: In this case, the efficiency is so small, or the power loss is so large that, among other things, the supplying of the field device with energy is no longer assured. This is problematic especially in the case of 4-20 mA field devices.
In EP 1 228 494 B1, reference is likewise made to the disturbance resistance of the transmission in the case of a cable variant of a field device. Here, interference removing means are placed in front of the outputs or the inputs of both processor units. The interference-removing means are lowpass filters, composed of a resistor and a capacitor, wherein the data lines are in each case grounded via the capacitor. The time constants of the RC members are selected in such a manner that, on the one hand, communication is not degraded, and, on the other hand, the coupling-in of interference is largely suppressed. Furthermore, the resistances are low ohm in such a way that an overly strong weakening of the signal level is prevented.
Disadvantageous in the case of the known solution is that it only enables a low frequency communication. Due to the lowpass arrangements, high-frequency communication is significantly degraded or completely impossible.